mongodb命令db.createCollection(name, options)創建一個新集合。由于 MongoDB 在命令中首次引用集合時會隱式創建集合，因此此方法主要用于創建使用特定選項的新集合。

例如，您使用db.createCollection()創建：固定大小集合；集群化集合；使用模式驗證的新集合。

db.createCollection()?方法具有以下原型形式：

db.createCollection( <name>,{capped: <boolean>,timeseries: {                  // Added in MongoDB 5.0timeField: <string>,        // required for time series collectionsmetaField: <string>,granularity: <string>,bucketMaxSpanSeconds: <number>,  // Added in MongoDB 6.3bucketRoundingSeconds: <number>  // Added in MongoDB 6.3},expireAfterSeconds: <number>,clusteredIndex: <document>,  // Added in MongoDB 5.3changeStreamPreAndPostImages: <document>,  // Added in MongoDB 6.0size: <number>,max: <number>,storageEngine: <document>,validator: <document>,validationLevel: <string>,validationAction: <string>,indexOptionDefaults: <document>,viewOn: <string>,pipeline: <pipeline>,collation: <document>,writeConcern: <document>})

db.createCollection()參數解釋：

mongodb源代碼src\mongo\db\commands文件夾下面是命令文件所在地：

count_cmd.cpp封裝count命令，distinct.cpp封裝了distinct命令，dbcommands.cpp封裝了CmdCreate和CmdDrop、CmdDatasize等。CmdCreate封裝了創建collection過程。CreateCommand解析create命令，重點是CreateCommand怎么來的？工具跟蹤進去是create_gen.cpp。create_gen.cpp原來是create.idl。

/* create collection */
class CmdCreate : public BasicCommand {
public:CmdCreate() : BasicCommand("create") {}virtual bool run(OperationContext* opCtx,const string& dbname,const BSONObj& cmdObj,BSONObjBuilder& result) {IDLParserErrorContext ctx("create");CreateCommand cmd = CreateCommand::parse(ctx, cmdObj);...}
} cmdCreate;

create.idl，idl文件是什么？

MongoDB 采用 IDL（接口定義語言）生成 C++ 代碼是一種常見的工程實踐，減少樣板代碼，提高開發效率，避免手動編寫重復邏輯（如字段提取、類型檢查、錯誤處理），確保代碼一致性（所有命令遵循相同的驗證規則）。

mongo\db\commands\create.idl內容是：

global:cpp_namespace: "mongo"imports:- "mongo/idl/basic_types.idl"commands:create:description: "Parser for the 'create' Command"namespace: concatenate_with_dbcpp_name: CreateCommandstrict: truefields:capped:description: "Specify true to create a capped collection. If you specify true, youmust also set a maximum size in the 'size' field."type: safeBooldefault: falseautoIndexId:description: "Specify false to disable the automatic creation of an index on the_id field."type: safeBooloptional: trueidIndex:description: "Specify the default _id index specification."type: objectoptional: truesize:...

create.idl怎么轉換成create.cpp的呢？

在 buildscripts 有一個目錄 idl，這里負責根據 src 中的 idl 生成文件。其中主要看buildscripts/idl/idl/generator.py文件，根據cpp_name生成對應的cpp文件，其中有一段邏輯：

def generate(self, spec):# type: (ast.IDLAST) -> None...spec_and_structs = spec.structsspec_and_structs += spec.commandsfor struct in spec_and_structs:self.gen_description_comment(struct.description)with self.gen_class_declaration_block(struct.cpp_name):self.write_unindented_line('public:')# Generate a sorted list of string constantsself.gen_string_constants_declarations(struct)self.write_empty_line()# Write constructorself.gen_class_constructors(struct)self.write_empty_line()# Write serializationself.gen_serializer_methods(struct)if isinstance(struct, ast.Command):self.gen_op_msg_request_methods(struct)# Write getters & settersfor field in struct.fields:if not field.ignore:if field.description:self.gen_description_comment(field.description)self.gen_getter(struct, field)if not struct.immutable and not field.chained_struct_field:self.gen_setter(field)if struct.generate_comparison_operators:self.gen_comparison_operators_declarations(struct)self.write_unindented_line('protected:')self.gen_protected_serializer_methods(struct)# Write private validatorsif [field for field in struct.fields if field.validator]:self.write_unindented_line('private:')for field in struct.fields:if not field.ignore and not struct.immutable and \not field.chained_struct_field and field.validator:self.gen_validators(field)self.write_unindented_line('private:')# Write command member variablesif isinstance(struct, ast.Command):self.gen_known_fields_declaration()self.write_empty_line()self.gen_op_msg_request_member(struct)# Write member variablesfor field in struct.fields:if not field.ignore and not field.chained_struct_field:self.gen_member(field)# Write serializer member variables# Note: we write these out second to ensure the bit fields can be packed by# the compiler.for field in struct.fields:if _is_required_serializer_field(field):self.gen_serializer_member(field)self.write_empty_line()for scp in spec.server_parameters:if scp.cpp_class is None:self._gen_exported_constexpr(scp.name, 'Default', scp.default, scp.condition)self._gen_extern_declaration(scp.cpp_vartype, scp.cpp_varname, scp.condition)self.gen_server_parameter_class(scp)if spec.configs:for opt in spec.configs:self._gen_exported_constexpr(opt.name, 'Default', opt.default, opt.condition)self._gen_extern_declaration(opt.cpp_vartype, opt.cpp_varname, opt.condition)self._gen_config_function_declaration(spec)

buildscripts/idl/idl/generator.py運行之后，python運行結果在對應的文件夾\build\opt\mongo\db\commands

create.idl生成了create_gen.h和create_gen.cpp，C++編譯之后create_gen.obj文件。

\build\opt\mongo\db\commands\create_gen.h，createCollection命令中的各個參數在下面文件都能看到，參數的get和set方法，代碼：

namespace mongo {/*** Parser for the 'create' Command*/
class CreateCommand {
public:...explicit CreateCommand(const NamespaceString nss);static CreateCommand parse(const IDLParserErrorContext& ctxt, const BSONObj& bsonObject);static CreateCommand parse(const IDLParserErrorContext& ctxt, const OpMsgRequest& request);void serialize(const BSONObj& commandPassthroughFields, BSONObjBuilder* builder) const;OpMsgRequest serialize(const BSONObj& commandPassthroughFields) const;BSONObj toBSON(const BSONObj& commandPassthroughFields) const;const NamespaceString& getNamespace() const { return _nss; }bool getCapped() const { return _capped; }void setCapped(bool value) & {  _capped = std::move(value);  }const boost::optional<bool> getAutoIndexId() const& { return _autoIndexId; }void getAutoIndexId() && = delete;void setAutoIndexId(boost::optional<bool> value) & {  _autoIndexId = std::move(value);  }
...

\build\opt\mongo\db\commands\create_gen.cpp，CreateCommand解析方法，createCollection命令解析成CreateCommand對象，代碼：

namespace mongo {...
CreateCommand::CreateCommand(const NamespaceString nss) : _nss(std::move(nss)), _dbName(nss.db().toString()), _hasDbName(true) {// Used for initialization only
}CreateCommand CreateCommand::parse(const IDLParserErrorContext& ctxt, const BSONObj& bsonObject) {NamespaceString localNS;CreateCommand object(localNS);object.parseProtected(ctxt, bsonObject);return object;
}CreateCommand CreateCommand::parse(const IDLParserErrorContext& ctxt, const OpMsgRequest& request) {NamespaceString localNS;CreateCommand object(localNS);object.parseProtected(ctxt, request);return object;
}

總結：buildscripts/idl/idl/generator.py把create.idl轉成create_gen.cpp和create_gen.h，再編譯成create_gen.obj，CreateCommand對象封裝命令createCollection。